I am having the same issue.
One thing to note though, when i initially ran the Skifta service in the ReadyNAS Ultra with an earlier version of the Firmware (Pre 4.2.15), Skifta service was running properly and I could see my place on my Android phone.
Once I upgraded the NAS firmware to version 4.2.15, I have having the exact same issue.
Noticed that you are also running the same firmware.
Any forum admins from Netgear who can advise on any bugs with this firmware number + skifta?
many thanks in advance,